Isaac K. Attah

Formation of nitrogen-containing polycyclic cations by gas-phase and intracluster reactions of acetylene with the pyridinium and pyrimidinium ions.

Here, we present evidence from laboratory experiments for the formation of nitrogen-containing complex organic ions by sequential reactions of acetylene with the pyridinium and pyrimidinium ions in the gas phase and within ionized pyridine-acetylene binary clusters. Additions of five and two acetylene molecules onto the pyridinium and pyrimidinium ions, respectively, at room temperature are observed. Second-order rate coefficients of the overall reaction of acetylene with the pyridinium and pyrimidinium ions are measured as 9.0 × 10(-11) and 1.4 × 10(-9) cm(3) s(-1), respectively, indicating reaction efficiencies of about 6% and 100%, respectively, at room temperature. At high temperatures, only two acetylene molecules are added to the pyridinium and pyrimidinium ions, suggesting covalent bond formation. A combination of ion dissociation and ion mobility experiments with DFT calculations reveals that the addition of acetylene into the pyridinium ion occurs through the N-atom of the pyridinium ion. The relatively high reaction efficiency is consistent with the absence of a barrier in the exothermic N-C bond forming reaction leading to the formation of the C(7)H(7)N(•+) covalent adduct. An exothermic addition/H-elimination reaction of acetylene with the C(7)H(7)N(•+) adduct is observed leading to the formation of a bicyclic quinolizinium cation (C(9)H(8)N(+)). Similar chemistry is observed in the sequential reactions of acetylene with the pyrimidinium ion. The second acetylene addition onto the pyrimidinium ion involves an exclusive addition/H-elimination reaction at room temperature leading to the formation of a bicyclic pyrimidinium cation (C(8)H(7)N(2)(+)). The high reactivity of the pyridinium and pyrimidinium ions toward acetylene is in sharp contrast to the very low reactivity of the benzene cation, which has a reaction efficiency of 10(-4)-10(-5). This indicates that the presence of a nitrogen atom within the aromatic ring enhances the ring growth mechanism by the sequential addition of acetylene to form nitrogen-containing polycyclic hydrocarbon ions. The observed reactions could explain the possible formation of nitrogen-containing complex organics by gas-phase ion-molecule reactions involving the pyridinium and pyrimidinium ions with acetylene under a wide range of temperatures and pressures in astrochemical environments such as the nitrogen-rich Titans atmosphere. The current results suggest searching for spectroscopic evidence for these organics in Titans atmosphere.

What Is the Structure of the Naphthalene–Benzene Heterodimer Radical Cation? Binding Energy, Charge Delocalization, and Unexpected Charge-Transfer Interaction in Stacked Dimer and Trimer Radical Cations

The binding energy of the naphthalene(+•)(benzene) heterodimer cation has been determined to be 7.9 ± 1 kcal/mol for C10H8(+•)(C6H6) and 8.1 ± 1 kcal/mol for C10H8(+•)(C6D6) by equilibrium thermochemical measurements using the mass-selected drift cell technique. A second benzene molecule binds to the C10H8(+•)(C6D6) dimer with essentially the same energy (8.4 ± 1 kcal/mol), suggesting that the two benzene molecules are stacked on opposite sides of the naphthalene cation in the (C6D6)C10H8(+•)(C6D6) heterotrimer. The lowest-energy isomers of the C10H8(+•)(C6D6) and (C6D6)C10H8(+•)(C6D6) dimer and trimer calculated using the M11/cc-pVTZ method have parallel stacked structures with enthalpies of binding (-ΔH°) of 8.4 and 9.0 kcal/mol, respectively, in excellent agreement with the experimental values. The stacked face-to-face class of isomers is calculated to have substantial charge-transfer stabilization of about 45% of the total interaction energy despite the large difference between the ionization energies of benzene and naphthalene. Similarly, significant delocalization of the positive charge is found among all three fragments of the (C6D6)C10H8(+•)(C6D6) heterotrimer, thus leaving only 46% of the total charge on the central naphthalene moiety. This unexpectedly high charge-transfer component results in activating two benzene molecules in the naphthalene(+•)(benzene)2 heterotrimer cation to associate with a third benzene molecule at 219 K to form a benzene trimer cation and a neutral naphthalene molecule. The global minimum of the C10H8(+•)(C6H6)2 heterotrimer is found to be the one where the naphthalene cation is sandwiched between two benzene molecules. It is remarkable, and rather unusual, that the binding energy of the second benzene molecule is essentially the same as that of the first. This is attributed to the enhanced charge-transfer interaction in the stacked trimer radical cation.

Formation of covalently bonded polycyclic hydrocarbon ions by intracluster polymerization of ionized ethynylbenzene clusters.

Here we report a detailed study aimed at elucidating the mechanism of intracluster ionic polymerization following the electron impact ionization of van der Waals clusters of ethynylbenzene (C8H6)n generated by a supersonic beam expansion. The structures of the C16H12, C24H18, C32H24, C40H30, and C48H36 radical cations resulting from the intracluster ion-molecule addition reactions have been investigated using a combination of mass-selected ion dissociation and ion mobility measurements coupled with theoretical calculations. Noncovalent structures can be totally excluded primarily because the measured fragmentations cannot result from noncovalent structures, and partially because of the large difference between the measured collision cross sections and the calculated values corresponding to noncovalent ion-neutral complexes. All the mass-selected cluster ions show characteristic fragmentations of covalently bonded molecular ions by the loss of stable neutral fragments such as CH3, C2H, C6H5, and C7H7. The population of the C16H12 dimer ions is dominated by structural isomers of the type (C6H5)-C≡C-CH(•+)CH-(C6H5), which can grow by the sequential addition of ethynylbenzene molecules, in addition to some contributions from cyclic isomers such as the 1,3- or 1,4-diphenyl cyclobutadiene ions. Similarly, two major covalent isomers have been identified for the C24H18 trimer ions: one that has a blocked cyclic structure assigned to 1,2,4- or 1,3,5-triphenylbenzene cation, and a second isomer of the type (C6H5)-C≡C-C(C6H5)═CH-CH(•+)CH-(C6H5) where the covalent addition of further ethynylbenzene molecules can occur. For the larger ions such as C32H24, C40H30, and C48H36, the major isomers present involve the growing oligomer sequence (C6H5)-C≡C-[C(C6H5)═CH]n-CH(•+)CH-(C6H5) with different locations and orientations of the phenyl groups along the chain. In addition, the larger ions contain another family of structures consisting of neutral ethynylbenzene molecules associated with the blocked cyclic isomer ions such as the diphenylcyclobutadiene and triphenylbenzene cations. Low-energy dissociation channels corresponding to evaporation of ethynylbenzene molecules weakly associated with the covalent ions are observed in the large clusters in addition to the high-energy channels corresponding to fragmentation of the covalently bonded ions. However, in small clusters only high-energy dissociation channels are observed corresponding to the characteristic fragmentation of the molecular ions, thus providing structural signatures to identify the product ions and establish the mechanism of intracluster ionic polymerization.

Substituent effects on noncovalent bonds: complexes of ionized benzene derivatives with hydrogen cyanide.

Here, we report the first experimental and computational study of the noncovalent binding energies and structures of ionized benzenes containing electron-withdrawing substituents solvated by one to four HCN molecules. Measured by ion mobility mass spectrometric equilibrium studies, the bond dissociation enthalpies of the first HCN molecule to the fluorobenzene (C6H5F(•+)), 1,4-difluorobenzene (C6H4F2(•+)), and benzonitrile (C6H5CN(•+)) ions (11.2, 11.2, and 9.2 kcal/mol, respectively) are similar to those of HCN with the benzene (C6H6(•+)) and phenyacetylene (C6H5CCH(•+)) radical cations (9.2 and 10.5 kcal/mol, respectively). DFT calculations at the B3LYP/6-311++G(d,p) level show that HCN can form in-plane hydrogen bonds to ring hydrogens, or bind electrostatically to positively charged carbon centers in the ring. The electron-withdrawing substituents increase the bond energy by increasing the partial positive charge on the ring hydrogens that form CH(δ+)---NCH bonds, and by creating a π hole, as evidenced by positive charge centers on the fluorinated ring carbons for electrostatically bonded isomers. In the complexes of benzonitrile(•+), similar to benzene(•+), hydrogen bonded planar isomers have the lowest energy. In the complexes of (fluorinated benzene)(•+), the lowest energy isomers are electrostatically bonded where HCN is perpendicular to the ring and its dipole points to a positively charged ring carbon. However, in all cases the planar hydrogen-bonded and vertical electrostatic isomers have similar binding energies within 1 kcal/mol, although HCN interacts with different sites of the ionized benzenes in these isomers, suggesting that the observed cluster populations are mixtures of the planar and vertical isomers. Further HCN molecules can bind directly to unoccupied ring CH hydrogens or bind to the first-shell HCN molecules to form linear HCN---HCN--- hydrogen bonded chains. The binding energies decrease stepwise to about 6-7 kcal/mol by 4 HCN molecules, approaching the macroscopic enthalpy of vaporization of liquid HCN (6.0 kcal/mol).

Communication: Ion mobility of the radical cation dimers: (Naphthalene)2+• and naphthalene+•-benzene: Evidence for stacked sandwich and T-shape structures

Dimer radical cations of aromatic and polycyclic aromatic molecules are good model systems for a fundamental understanding of photoconductivity and ferromagnetism in organic materials which depend on the degree of charge delocalization. The structures of the dimer radical cations are difficult to determine theoretically since the potential energy surface is often very flat with multiple shallow minima representing two major classes of isomers adopting the stacked parallel or the T-shape structure. We present experimental results, based on mass-selected ion mobility measurements, on the gas phase structures of the naphthalene(+⋅) ⋅ naphthalene homodimer and the naphthalene(+⋅) ⋅ benzene heterodimer radical cations at different temperatures. Ion mobility studies reveal a persistence of the stacked parallel structure of the naphthalene(+⋅) ⋅ naphthalene homodimer in the temperature range 230-300 K. On the other hand, the results reveal that the naphthalene(+⋅) ⋅ benzene heterodimer is able to exhibit both the stacked parallel and T-shape structural isomers depending on the experimental conditions. Exploitation of the unique structural motifs among charged homo- and heteroaromatic-aromatic interactions may lead to new opportunities for molecular design and recognition involving charged aromatic systems.

Proton-bound dimers of nitrogen heterocyclic molecules: Substituent effects on the structures and binding energies of homodimers of diazine, triazine, and fluoropyridine

The bonding energies of proton-bound homodimers BH(+)B were measured by ion mobility equilibrium studies and calculated at the DFT B3LYP/6-311++G** level, for a series of nitrogen heterocyclic molecules (B) with electron-withdrawing in-ring N and on-ring F substituents. The binding energies (ΔH°(dissoc)) of the proton-bound dimers (BH(+)B) vary significantly, from 29.7 to 18.1 kcal/mol, decreasing linearly with decreasing the proton affinity of the monomer (B). This trend differs significantly from the constant binding energies of most homodimers of other organic nitrogen and oxygen bases. The experimentally measured ΔH°(dissoc) for (1,3-diazine)2H(+), i.e., (pyrimidine)2H(+) and (3-F-pyridine)2H(+) are 22.7 and 23.0 kcal/mol, respectively. The measured ΔH°(dissoc) for the pyrimidine(·+)(3-F-pyridine) radical cation dimer (19.2 kcal/mol) is signifcantly lower than that of the proton-bound homodimers of pyrimidine and 3-F-pyridine, reflecting the stronger interaction in the ionic H-bond of the protonated dimers. The calculated binding energies for (1,2-diazine)2H(+), (pyridine)2H(+), (2-F-pyridine)2H(+), (3-F-pyridine)2H(+), (2,6-di-F-pyridine)2H(+), (4-F-pyridine)2H(+), (1,3-diazine)2H(+), (1,4-diazine)2H(+), (1,3,5-triazine)2H(+), and (pentafluoropyridine)2H(+) are 29.7, 24.9, 24.8, 23.3, 23.2, 23.0, 22.4, 21.9, 19.3, and 18.1 kcal/mol, respectively. The electron-withdrawing substituents form internal dipoles whose electrostatic interactions contribute to both the decreased proton affinities of (B) and the decreased binding energies of the protonated dimers BH(+)B. The bonding energies also vary with rotation about the hydrogen bond, and they decrease in rotamers where the internal dipoles of the components are aligned efficiently for inter-ring repulsion. For compounds substituted at the 3 or 4 (meta or para) positions, the lowest energy rotamers are T-shaped with the planes of the two rings rotated by 90° about the hydrogen bond, while the planar rotamers are weakened by repulsion between the ortho hydrogen atoms of the two rings. Conversely, in ortho-substituted (1,2-diazine)2H(+) and (2-F-pyridine)2H(+), attractive interactions between the ortho (C-H) hydrogen atoms of one ring and the electronegative ortho atoms (N or F) of the other ring are stabilizing, and increase the protonated dimer binding energies by up to 4 kcal/mol. In all of the dimers, rotation about the hydrogen bond can involve a 2-4 kcal/mol barrier due to the relative energies of the rotamers.

Distinguishing enantiomeric amino acids with chiral cyclodextrin adducts and structures for lossless ion manipulations

Enantiomeric molecular evaluations remain an enormous challenge for current analytical techniques. To date, derivatization strategies and long separation times are generally required in these studies, and the development and implementation of new approaches are needed to increase speed and distinguish currently unresolvable compounds. Herein, we describe a method using chiral cyclodextrin adducts and structures for lossless ion manipulations (SLIM) and serpentine ultralong path with extended routing (SUPER) ion mobility (IM) to achieve rapid, high resolution separations of d and l enantiomeric amino acids. In the analyses, a chiral cyclodextrin is added to each sample. Two cyclodextrins were found to complex each amino acid molecule (i.e. potentially sandwiching the amino acid in their cavities) and forming host‐guest noncovalent complexes that were distinct for each d and l amino acid pair studied and thus separable with IM in SLIM devices. The SLIM was also used to accumulate much larger ion populations than previously feasible for evaluation and therefore allow enantiomeric measurements of higher sensitivity, with gains in resolution from our ultralong path separation capabilities, than previously reported by any other IM–based approach.

Noncovalent Interactions of Organic Ions with Polar Molecules in the Gas Phase

The chapter is focused on noncovalent interactions of organic ions with small polar molecules in the gas phase. The organic ions studied include cyclic C3H3 + and the radical cations of benzene (C6H6 •+), pyridine (C5NH5 •+), pyrimidine (C5N2H4 •+), fluorobenzene (C6H5F•+), phenylacetylene (C8H6 •+), benzonitrile (C7NH5 •+) and naphthalene (C10H8 •+). In addition, protonated pyridine (pyridine.H+) and protonated pyrimidine (pyrimidine.H+) are also included for comparison with the radical cations. The solvent molecules include water (H2O), hydrogen cyanide (HCN) and acetonitrile (CH3CN). The results presented include experimental thermochemical data (ΔH° and ΔS°) for the stepwise association of the solvent molecules with the organic ions and theoretically calculated binding energies and structures. The four major topics discussed are: (1) Trends in binding energies and entropy changes, (2) Ionic hydrogen bonds with organic ions, (3) Internal vs. external solvation of the organic ions, and (4) Intracluster proton transfer and deprotonation of the organic ions.